An ever-increasing demand for higher system thermal efficiency has necessitated the operation of power generation cycles and heat conversion systems for chemical processes at progressively higher temperatures. As system operating temperatures are increased, fewer materials with acceptable mechanical properties and environmental compatibility are known. This dearth of materials is particularly acute in applications at temperatures above 650° C. and at significant stress levels where liquid fluoride salts are favored as heat transfer media because of their high thermal capacity and low vapor pressure. There is therefore a need for structural alloys for high-temperature heat transfer applications in order to enable increased thermal efficiency of energy conversion and transport systems thereby reducing system costs as well as reducing the waste heat rejected to the environment.
Fluoride salt cooled High temperature Reactors (FHRs) potentially have attractive performance and safety attributes. Defining features of FHRs include coated particle fuel, low-pressure fluoride salt cooling, and high-temperature heat production. The FHR heat transfer technology base is derived primarily from earlier molten salt reactors and their coated particle fuel is similar to that developed for high-temperature helium-cooled reactors. The excellent heat transfer characteristics of liquid fluoride salts enable full passive safety, at almost any power scale thereby enabling large power output reactors, with less massive piping and containment structures, and consequent economies of scale. FHRs potentially have improved economics, increased safety margins, and lower water usage characteristics than conventional water-cooled reactors.
The fuel and coolants for FHRs are suitable for operation at temperatures well in excess of the upper temperature limits of available structural alloys. A limiting factor in achieving the highest possible FHR core outlet temperatures and thus thermal efficiency is the availability of structural alloys having sufficient creep strength at the required temperatures combined with suitable fluoride salt chemical compatibility as well as ease of fabrication. Hastelloy® N (trademark owned by Haynes International, Inc.) (also known as Alloy N and INOR-8), developed at Oak Ridge National laboratory (ORNL) in the 1950s and 1960s, is currently a leading candidate FHR structural alloy for operations below 700° C. Alloy N is limited to use in low stress applications to a maximum temperature of about 704° C. due to insufficient creep strength at higher temperatures, is limited to use in high stress applications such as steam generator tubes to about 600° C. due to insufficient creep strength at higher temperatures, is not fully qualified to current code requirements for high temperature reactors, and is challenging to fabricate due to its work hardening characteristics. There is therefore a need for corrosion-resistant nickel-based structural alloys designed to possess good creep resistance in liquid fluorides at higher temperatures in order to provide substantial improvements in FHR economics and performance. Calculations reveal that a net thermal efficiency of greater than 50% (as compared to about 33% net thermal efficiency of existing reactors) would be likely for FHRs using a high temperature structural alloy with concurrent reductions in capital costs, waste generation, fissile material requirements, and cooling water usage.
In general, conventional Ni-based alloys are strengthened through a combination of solid solution strengthening and precipitation strengthening mechanisms with the latter needed to achieve higher strengths at higher temperatures. In one class of Ni-based superalloys, primary strengthening is obtained through the homogeneous precipitation of ordered, L12 structured, Ni3(AI,Ti,Nb)-based intermetallic precipitates that are coherently embedded in a solid solution FCC matrix. In another class of Ni-based alloys, creep resistance is achieved through the precipitation of fine carbides (M23C6, M7C3, M6C where M is primarily Cr with substitution of Mo, W, for example) and carbonitrides (M(C, N) where M is primarily Nb, or Ti, for example) within the matrix, and larger carbides on grain boundaries to prevent grain boundary sliding. Moreover, high temperature oxidation resistance in these alloys is obtained through additions of Cr and Al. Existing data (shown in FIG. 1) on liquid fluoride salt resistance of Ni-based alloys show that alloys containing aluminum, and substantial amounts of chromium have lower resistance to liquid fluoride salt. Commercial Nickel-based alloys with high strengths typically contain significant amounts of Cr (greater than 15 wt. % Cr) making them unsuitable for use in contact with liquid fluoride salts. Compositions (in weight %) of several commercially produced Ni-based alloys strengthened by γ′ precipitation are shown in Table 1.
Hastelloy® N is an alloy that was designed to balance resistance to liquid fluoride salt corrosion with good creep properties at temperatures up to 704° C. This alloy is a Ni—Mo alloy containing additional alloying elements with solid solution strengthening being the primary strengthening mechanism; Hastelloy® N does not have γ′ precipitation strengthening. Its nominal composition is given as71Ni−7Cr−16Mo−5Fe*−1Si*−0.8Mn*−0.2Co*−0.35Cu*−0.5W*−0.35Al+Ti*−0.08C*
where * indicates maximum allowed content of the indicated elements. Hastelloy® N generally consists of the following elements to provide the corresponding benefits:
Chromium: Added to ensure good oxidation resistance but minimized to keep liquid fluoride salt corrosion within acceptable limits. Also provides solid solution strengthening. Too much addition results in excessive attack by liquid fluoride salts.
Molybdenum: Principal strengthening addition for solid solution strengthening, provides good resistance to liquid fluoride salt, and results in lower interdiffusion coefficients. Also is the primary constituent in M6C carbides. Too much addition can result in the formation of undesirable, brittle intermetallic phases.
Iron: Provides solid solution strengthening. Too much addition can destabilize austenitic matrix and decrease resistance to liquid fluoride salt.
Manganese: Stabilizes the austenitic matrix phase. Provides solid solution strengthening.
Silicon: Assists in high temperature oxidation resistance, a maximum of 1% Si may be added.
Carbon, Nitrogen: Required for the formation of carbide and/or carbonitride phases that can act as grain boundary pinning agents to minimize grain growth and to provide resistance to grain boundary sliding. Fine precipitation of carbide and/or carbonitride phases can increase high temperature strength and creep resistance.
Copper: Stabilizes the austenitic matrix, provides solid solution strengthening.
Cobalt: Provides solid solution strengthening.
Tungsten: Provides solid solution strengthening and decreases average interdiffusion coefficient. Too much can result in the formation of brittle intermetallic phases that can be deleterious to processability.
Aluminum+Titanium are not desirable in Hastelloy® N, in order to minimize corrosion by liquid salt. Combined wt. % of Al+Ti is typically kept to less than 0.35.
FIG. 1 shows effects of alloying element additions on the depth of corrosion of Ni-alloys in 54.3LiF-41.0KF-11.2NaF-2.5UF4 (mole percent) in a thermal convention loop operated between 815 and 650° C. (smaller depth of corrosion is better).
FIG. 2 shows the equilibrium phase fractions in Hastelloy® N as a function of temperature. Note that solid solution strengthening and some carbide strengthening (through M6C) are the primary strengthening mechanisms active in Hastelloy® N. This limits the strength and creep resistance of Hastelloy® N at high temperatures and restricts its useful temperatures to less than about 704° C. Components such as secondary heat exchangers need to withstand large pressure differences between salt on one side of the heat exchanger wall and a gaseous fluid at higher pressures on the other side. Such components hence need materials with high temperature strength greater than that of Hastelloy® N along with good resistance to salt, good oxidation resistance, and in the case of FHRs, tolerance to nuclear irradiation.